The invention relates to production methods for mirror elements and to mirror elements comprising a reflective coating for the EUV wavelength range and a substrate.
It is known that the density of many materials, especially materials containing silicon, changes under irradiation with high-energy radiation. That effect is referred to in the literature as “compaction”. For applications in extreme environments (reactors, outer space) in particular, studies were carried out and those effects were quantitatively determined a long time ago (see W. Primak, Nucl. Sci. Eng., 65, 141, 1978: “Radiation Behaviour of Vitreous Silica” and R. A. B. Devine, “Macroscopic and Microscopic Effects of Radiation in Amorphous SiO2”, Nuclear Instruments and Methods in Physics Research B 91 (1994), 378-390).
It has been found that the change in volume or density in silicon dioxide typically attains, after sufficiently long irradiation, a saturation value on the order of about 2%-3% within the penetration depth reached by the radiation. The penetration depths of the high-energy types of radiation considered therein were typically in a range of from 0.5 μm to about 10 μm or more.
Comparable effects are also known in microlithography, especially for the VUV wavelength range; however, owing to the relatively low level of interaction of the VUV light with the optical material used, especially quartz glass, the changes in volume in that case are as a rule in the ppm range and therefore, typically, no saturation value is attained, the optical materials used in that case being completely penetrated by the radiation.
A method is known from U.S. Pat. No. 6,205,818 B1 by which quartz glass (SiO2) is to be made insensitive to compacting caused by long-term irradiation with UV laser radiation. The method provides for the quartz glass material to be pre-compacted by being exposed to high-energy radiation or by being pre-treated by hot isostatic pressing (HIP). The high-energy radiation is said to make a compaction of between about 10 ppm and 100 ppm possible, while a change in volume of the entire quartz glass body of from about 0.1% to about 3% is said to be achievable by hot isostatic pressing.
Since microlithography will have to rely in future on the EUV wavelength range in order to obtain a further increase in resolution and since, owing to their coating, the mirrors used in that case are capable of reflecting only about 70% of the incident light and consequently absorb about 30% of the incident light, materials having a low coefficient of thermal expansion are normally used as substrate material for such mirrors. Such so-called “low expansion materials” are, for example, Zerodur®, ULE® or Clearceram®. Those materials normally have an amorphous silicate glass content of above about 50% and, in extreme cases, of even 100%. For a projection exposure system to be capable of functioning on a long-term basis it is necessary to ensure, therefore, that the energy absorbed in the substrate material during operation does not lead to changes in the substrate and thus to degradation of the mirror surface. In other words, it is necessary to ensure that changes of any kind in the shape or roughness of the surface, which can lead to a no longer tolerable increase in aberrations or stray light, do not occur.